The present invention relates to an improvement in an illumination responsive lamp control circuit, and particularly, in a delay device for delaying the response of the circuit to changes in illumination.
It is desirable to delay the response of a lamp control circuit to a change in illumination to avoid response to certain transient illumination conditions, and additionally, to avoid lamp flicker at threshold illumination levels, i.e., at illumination levels near the threshold for the energization/de-energization of the lamp.
By way of practical examples illustrating the need for such delay in the control of the lamp, consider the use of an illumination responsive headlamp control for an automobile. It is desirable to keep the headlamps on during the night time period even though the illumination sensor may be temporarily exposed to street lamp illumination above the threshold level. Furthermore, it is desirable to keep the headlamps off during the day time period even during intermittent low levels of illumination encountered within tunnels or beneath underpasses. Even in other applications such as outdoor lighting, both decorative and functional, a delay in the control of the lamp is desirable. Consider, for example, a decorative post lamp which is periodically illuminated by headlights. A delay circuit could prevent spurious response to headlamps.
The usual method for providing such a time delay is to incorporate a capacitor time-delay circuit. Such a circuit would include a capacitor, a source of charging current for the capacitor, a discharging path for the capacitor and a threshold device such as a unijunction transistor. It will be appreciated that capacitor time-delay circuits are relatively expensive to produce and are complex in their structure and operation. As a result of their complexity, construction costs are significant and the reliability of the circuit is adversely affected.
The present invention provides a simple, compact, low cost and reliable delay device for a lamp control circuit. The circuit requires no moving parts such as relays and the like, and does not require relatively expensive capacitors and threshold devices such as unijunction transistors.
According to the present invention, a delay device is interposed between the illumination sensor and the controlled conduction device, e.g., an SCR or triac, which features thermal transfer of a signal from the illumination sensor to the controlled conduction device. For example, a delay circuit may include a heater/resistor connected in series with the illumination sensor so that the thermal energy output of the heater/resistor is a function of the illumination level. The heater/resistor is in thermal transfer relationship with a temperature responsive device such as a thermistor. The thermistor is responsive to the thermal energy emitted by the heater/resistor and is connected to the controlled conduction device to control the conduction thereof in accordance with received thermal energy. Since a finite period of time is required for the heater/resistor to either increase its temperature or decrease its temperature in response to a change of illumination level, since a further finite period of time is required for thermal transfer between the thermistor and the heater/resistor, and since a still further finite period of time is required for the temperature of the thermistor to change to thereby change the control signal supplied to the controlled conduction device, the heater/resistor and thermistor combination introduces a time delay in the response of the controlled conduction device to a change in the illumination level which is a function of all three delays. The delay period can be adjusted by adjusting the thermal output of the heater/resistor, the temperature sensitivity of the thermistor, the thermal capacity of the thermal transfer structure associated with the heater/resistor and thermistor, and the length and conductivity of the thermal path of such thermal transfer structure. These factors can be controlled to introduce a time delay of appropriate duration.